Evolutionary Glitch Behind Backbone Fractures Discovered

Osteoporosis might not be the only culprit behind the increased risk of
backbone fractures as we age. Researchers at the School of Medicine have
discovered that the very musculoskeletal features that evolved to let humans
walk upright might also contribute to weaker spines.

Compared to apes, humans have larger, more porous vertebrae encased
in a much thinner shell of bone. Just like broad heel bones and broad ends of
the leg bones in humans, these vertebrae are specially designed for upright
walking—they dissipate the impact of treading on two feet, protecting spinal
cartilage and discs.

The design works well until men and women age and suffer natural bone
loss—leaving them vulnerable to fractures and breaks, researchers say.
“In evolution, we have a great adaptation, but there is sometimes a tradeoff,”
says Meghan Cotter, PhD, an anatomy instructor at the medical school
and lead author of the study. “The structure is great for walking around, but
not good when you have osteoporosis.”

Apes, on the other hand, have much thicker vertebral shells to begin with—
designed to provide stability for climbing and knuckle-walking. They can
suffer comparable bone loss with age, but their vertebrae remain intact.
A major reorganization of the musculoskeletal system was required for
early human ancestors to make the transition from walking on all fours to
walking upright, the researchers say.

“We’re now living about twice as long as when the adaptation evolved,
and that results in major problems,” Cotter says. “It highlights we are not
perfectly evolved specimens.”